11.2 Plant Gametes and Fertilization
Key Takeaways
In the anther, meiosis of a microspore mother cell yields four haploid microspores, and each can develop into a pollen grain.
A pollen grain is the male gametophyte and has a tube cell plus a generative cell that produces two sperm by mitosis.
In the ovule, one surviving megaspore becomes the embryo sac, classically seven cells and eight nuclei, including the egg and two polar nuclei.
Pollination transfers pollen to a stigma, and fertilization is the later fusion of gametes after the pollen tube reaches the embryo sac.
Double fertilization forms a diploid zygote and triploid endosperm, the ovule becomes the seed, and the ovary becomes the fruit.
11.2 Plant Gametes and Fertilization
In a flowering plant the sporophyte is the large body, and the gametophytes are tiny. Meiosis inside the anther and the ovule produces spores. Mitosis then builds the gametophytes that make the gametes. This section follows those nuclei through pollination to double fertilization. Flower parts such as the stamen, carpel, stigma, and ovary are the map. The new work is which division happens in each place, and which nucleus is haploid, diploid, or triploid.
From a microspore mother cell to pollen
A diploid microspore mother cell in the anther undergoes meiosis. The four products are haploid microspores. Each microspore develops into a pollen grain by mitosis and differentiation, not by another meiosis. The finished grain is the male gametophyte. It is no longer a spore. A typical grain has a protective wall, a tube cell, also called the vegetative cell, and a smaller generative cell.
Two sperm
The generative cell divides by mitosis and produces two sperm. In some species that happens before the grain is shed. In others it happens in the pollen tube after the grain lands. Either way, the two sperm are haploid, and both move with the tube. The meiosis that set their chromosome number was the division of the microspore mother cell. Sperm formation itself is mitosis.
From a megaspore mother cell to the embryo sac
An ovule contains a diploid megaspore mother cell. Meiosis produces four haploid megaspores. In the usual pattern, three degenerate and one survives, commonly the megaspore farthest from the micropyle, the small opening of the ovule. The survivor is still a spore, not an egg. It must build the female gametophyte.
Eight nuclei, seven cells
The surviving megaspore divides by mitosis three times. The product is the embryo sac, classically eight nuclei arranged as seven cells. At the micropyle end are the egg and two synergids. At the far end are three antipodal cells. The large central cell contains the two polar nuclei. Six of the cells have one nucleus each, and the central cell has two, so the sac holds eight nuclei in seven cells. The egg is haploid. Each polar nucleus is haploid. The sac is not diploid, because fertilization has not happened.
Pollination only moves the pollen
Pollination is the transfer of pollen from an anther to a stigma. Wind, water, or an animal may carry it. Arrival on the stigma is the end of pollination. It is not the joining of sperm and egg. A grain can land and still fail to fertilize an egg if the tube stops, the species is incompatible, or the flower is too old.
A compatible grain germinates. The tube cell grows a pollen tube through the style, into the ovary, and into an ovule, usually by way of the micropyle. The two sperm travel near the tip. Signals from the synergids help guide the tube to the embryo sac, and the tube then releases the sperm. Only then can fusion occur.
What each sperm does
Flowering plants use both sperm, and the two fusions have different products. One sperm joins the egg and forms the diploid zygote, the first cell of the embryo and of the next sporophyte. The other sperm joins the two polar nuclei. Three haploid sets in one nucleus make a triploid nucleus, which divides into endosperm, the food tissue of the seed. Together these fusions are double fertilization. Conifer fertilization produces a seed after one sperm joins an egg, but it does not include a second fertilization that makes triploid endosperm; the food tissue in a conifer seed is haploid female-gametophyte tissue.
After the fusions, the ovule matures into the seed. The zygote becomes the embryo. Endosperm feeds that embryo. In many eudicots the cotyledons take up most of the endosperm, so a mature bean stores food in two cotyledons, while a corn kernel keeps a large endosperm next to its single cotyledon. The seed coat comes from the integuments of the ovule, not from either sperm fusion. The ovary matures into the fruit. Fruit wall is parent sporophyte tissue wrapped around the seed. It is neither the embryo nor the endosperm.
The angiosperm sequence
| Step | Place | Result |
|---|---|---|
| Microspore mother cell meiosis | Anther | Four haploid microspores |
| Development of each microspore | Pollen grain | Male gametophyte with a tube cell and a generative cell |
| Generative-cell mitosis | Grain or pollen tube | Two haploid sperm |
| Megaspore mother cell meiosis | Ovule | Four megaspores, one of which usually survives |
| Mitosis of the surviving megaspore | Embryo sac | Seven cells and eight nuclei, with the egg and two polar nuclei |
| Pollination | Anther to stigma | Pollen is transferred, and gametes have not fused |
| Double fertilization | Embryo sac | Diploid zygote and triploid endosperm |
| Maturation | Ovule and ovary | The ovule becomes the seed, and the ovary becomes the fruit |
Important
A pollen grain is already the male gametophyte, not a spore that still must germinate into one. Pollination only carries that gametophyte to a stigma. Fertilization is the later fusion inside the embryo sac.
Keeping the counts straight
The microspore mother cell is diploid. The pollen grain and the embryo sac are haploid gametophytes. The zygote is the first diploid cell of the new sporophyte, and endosperm is a separate triploid tissue that does not make gametes. The seed coat and the fruit wall are parent tissues. They surround the embryo. They are not products of sperm fusion.
A grain that shows a tube cell and a generative cell is already the male gametophyte. Pollen on a stigma is pollination. Fertilization is scored only when a sperm fuses inside the embryo sac. If an item asks which tissue is triploid, the tissue is endosperm.
A microspore mother cell in a lily anther completes meiosis. What happens next to its products?
One megaspore survives and becomes an embryo sac that contains eight sperm.
The mother cell fuses with a polar nucleus and becomes a fruit.
Two diploid sperm form immediately and are released as the entire male gametophyte.
Four haploid microspores form, and each can develop into a pollen grain, the male gametophyte, with a tube cell and a generative cell that produces two sperm.
Double fertilization has occurred in an ovule. Which products are formed, and from which nuclei?
A microspore and a megaspore fuse on the stigma and form the fruit wall.
Both sperm join the egg, forming two diploid embryos and no nutritive tissue.
One sperm and the egg form triploid endosperm, and the other sperm and a synergid form the seed coat.
One sperm and the egg form a diploid zygote, and the other sperm and the two polar nuclei form triploid endosperm.
Pollen from one tomato flower lands on the stigma of another tomato flower. Which statement is accurate?
Pollination has delivered the male gametophyte to the stigma, but fertilization happens later, when a sperm from the pollen tube fuses with the egg in the embryo sac.
Fertilization is already finished because the pollen grain is a spore that fused with the egg on the stigma.
The pollen grain must still undergo meiosis on the stigma before it counts as a gametophyte.
Pollination and fertilization are the same fusion, and both are complete when pollen touches the stigma.
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